BIOMASS ACCUMULATION, NITROGEN FIXATION AND GRAIN YIELD OF SOYBEAN (Glycine max) VARIETIES INTERCROPPED WITH MAIZE (Zea mays) AT THREE PLANTING DENSITIES IN TWO CONTRASTING SOILS OF WESTERN KENYA

Abstract Biomass accumulation, nitrogen fixation and grain yields in soybean grown as intercrops with maize at different planting densities was assessed in Butere and Mumias districts of western Kenya with humic nitosols and orthic ferralsols respectively during the short rains of 2011 and long rains of 2012. A RCBD with a factorial arrangement replicated thrice was used. Three soybean varieties SB 25 (NAMSOY 4m), SB 132 (SC SQUIRE) and SB 118 (TGx 1987-18F) were intercropped with imidazolinone- resistant maize (IR maize). Three maize: soybean planting densities, 1:1 (D1), 1:2 (D2) and 1:3 (D3) and sole soybean (D4) were tested. Above ground biomass (ABGM) was determined by destructive sampling of plants, nitrogen fixation assessed using the 15N natural abundance method and yields assessed from the net plot at harvest. Soybean AGBM yields increased with increase in soybean planting densities across seasons and sites. There were significant differences in AGBM (P 0.001) across planting densities except for soybean variety NAMSOY 4m. D3, for soybean variety TGx 1987-18F produced the highest AGBM (3759.2 and 2476.7kg/ha) in Butere district and (2176.8 and 1645kg/ha) in Mumias district during the short and long rain season respectively. Nitrogen fixation increased with the increase in biomass across varieties, districts and seasons. Significant differences in N fixation (P0.001) were also detected, with D3 for soybean variety TGx 1987-18F recording the highest N fixation (104.2 and 72.6kg/ha) in Butere district and (57.2 and 46.4kg/ha) in Mumias district in the short and long rain season respectively. Grain yields were similarly significantly different across planting densities (P 0.05) with D3 for soybean variety SC SQUIRE recording the highest grain yield among the intercrops (1,192.3 and 724.2kg/ha) in the short rain season and (1,839.4 and 1,726.4kg/ha) in the long rain season in Butere and Mumias districts respectively. TGx 1987-18F emerged the best variety in terms of biomass accumulation and N fixation however, to build soil fertility while at the same time maximizing soybean yields, D3 for soybean variety SC SQUIRE planted in the short rain season is worth adopting in the smallholder farming system of western Kenya. Keywords: Planting densities, biomass, soybean genotypes, N accumulation, 15N natural abundance.